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X-ray vision

3 October 1998

By Marcus Chown

PULSARS that only emit X-rays, once considered “anomalous”, now officially
outnumber those that emit radio waves. This is leading astronomers to rethink
their ideas about what happens after a typical dying star explodes as a
supernova.

A supernova explosion occurs when a star runs out of nuclear fuel and shrinks
catastrophically under its own gravity. The result is a super-dense neutron star
about the size of Mount Everest. Current theories of how these stars behave
predict that they should all act as radio pulsars, sweeping a narrow beam of
radio waves around the sky like a lighthouse a hundred times a second. So why
didn’t radio searches find more pulsars in supernova remnants?

“The trouble is that hardly more than 1 per cent of the 300-odd known young
supernova remnants contain associated radio pulsars,” says Eric Gotthelf of
NASA’s Goddard Space Flight Center near Washington DC. But now the reason they
went missing is clear, Gotthelf says. Astronomers were looking in the wrong part
of the electromagnetic spectrum.

In the past few years, astronomers using the Japanese-American ASCA satellite
have found three “point-like” objects in the centres of supernova remnants which
are emitting pulses of X-rays. Now Gotthelf says that he has just picked out
three more of these “anomalous X-ray pulsars” (AXPs) in X-ray sources observed
by the satellite. Add these three to the list and anomalous pulsars in
supernovae remnants will outnumber the four known radio pulsars associated with
supernovae remnants for the first time, he reports in a paper to appear in the
journal Memorie della Societá Astronomica Italiana.

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These findings will mean that radio pulsars are the exception rather than the
norm. “This is a complete reversal of our thinking,” says Gotthelf. David Hough
of Trinity University in San Antonio, Texas, confirms that these new findings
mean that “the book on how pulsars are born in supernovae may have to be
rewritten”.

The X-rays coming from AXPs are produced by matter channelled by the star’s
magnetic field lines and heated to enormous temperatures. AXPs spin a thousand
times slower than radio pulsars and are slowing down rapidly. This is puzzling,
because when a star shrinks to the relatively tiny size of a neutron star it
should automatically spin very fast. According to Gotthelf, the most likely
explanation is that AXPs are indeed born spinning fast, but slow down quickly
because they have a super-strong magnetic field, hundreds of times stronger than
in radio pulsars.

“Such a strong magnetic field would drag material around as the star spins,
sapping the star of rotational energy,” says Gotthelf. A super-strong magnetic
field would also prevent the formation of the electrons needed to produce radio
waves. One possible explanation for the different magnetic field strengths in
radio pulsars and X-ray pulsars is that stars start out with a natural
variability in magnetic fields before they collapse.

At least two more sensitive X-ray satellites will be launched in the next few
years, and Gotthelf believes they will find many more radio-quiet pulsars.